Docstoc

Eye Color

Document Sample
Eye Color Powered By Docstoc
					Eye color
Eye color is a polygenic phenotypic character and is determined by the amount and type
of pigments in the eye's iris.[1][2] Humans and other animals have many phenotypic
variations in eye color, as blue, brown, gray, green, and others. These variations
constitute phenotypic traits.[3]

The genetics of eye color are complicated, and color is determined by multiple genes.
Some of the eye-color genes include EYCL1 (a green/blue eye-color gene located on
chromosome 19), EYCL2 (a brown eye-color gene) and EYCL3 (a brown/blue eye-color
gene located on chromosome 15). The once-held view that blue eye color is a simple
recessive trait has been shown to be incorrect. The genetics of eye color are so complex
that almost any parent-child combination of eye colors can occur.[4][5]

In human eyes, these variations in color are attributed to varying ratios of eumelanin
produced by melanocytes in the iris.[2] The brightly colored eyes of many bird species are
largely determined by other pigments, such as pteridines, purines, and carotenoids.[6]

Three main elements within the iris contribute to its color: the melanin content of the iris
pigment epithelium, the melanin content within the iris stroma, and the cellular density of
the iris stroma.[7] In eyes of all colors, the iris pigment epithelium contains the black
pigment, eumelanin.[2][7] Color variations among different irides are typically attributed to
the melanin content within the iris stroma.[7] The density of cells within the stroma affects
how much light is absorbed by the underlying pigment epithelium.[7] OCA2 gene
polymorphism, close to proximal 5′ regulatory region, explains most human eye-color
variation.[8]

[edit] Genetic determination of eye color
See also: Human genetic clustering

Eye colors can range from the most common color, brown, to the least common, green.
Eye color is an inherited trait influenced by more than one gene.[9][10] These genes are
being sought using associations to small changes in the genes themselves and in
neighboring genes. These changes are known as single-nucleotide polymorphisms or
SNPs. The actual number of genes that contribute to eye color is currently unknown, but
there are a few likely candidates. A study in Rotterdam (2009) found that it was possible
to predict the color of eyes with more than 90% accuracy for brown and blue, using just
six SNPs (from six genes).[11]

The gene OCA2 (OMIM: 203200), when in a variant form, causes the pink eye color and
hypopigmentation common in human albinism. (The name of the gene is derived from
the disorder it causes, oculocutaneous albinism type II.) Different SNPs within OCA2 are
strongly associated with blue and green eyes as well as variations in freckling, mole
counts, hair and skin tone. The polymorphisms may be in an OCA2 regulatory sequence,
where they may influence the expression of the gene product, which in turn affects
pigmentation.[8] A specific mutation within the HERC2 gene, a gene that regulates OCA2
expression, is partly responsible for blue eyes.[12] Other genes implicated in eye color
variation are: SLC24A4[13] and TYR.[13]

Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different
part of the genome. As Eiberg said: "The SNP rs12913832 [of the Herc2 gene] is found
to be associated with the brown and blue eye color, but this single DNA variation cannot
explain all the brown eye color variation from dark brown over hazel to blue eyes with
brown spots."

[edit] Classification of color
Iris color can provide a large amount of information about an individual, and a
classification of various colors may be useful in documenting pathological changes or
determining how a person may respond to various ocular pharmaceuticals.[14] Various
classification systems have ranged from a basic light or dark description to detailed
gradings employing photographic standards for comparison.[14] Others have attempted to
set objective standards of color comparison.[15]

As the perception of color depends on viewing conditions (e.g., the amount and kind of
illumination, as well as the hue of the surrounding environment), so does the perception
of eye color.[16]

Eye colors range from the darkest shades of brown to the lightest tints of blue.[9] To meet
the need for standardized classification, at once simple yet detailed enough for research
purposes, Seddon et al. developed a graded system based on the predominant iris color
and the amount of brown or yellow pigment present.[17] There are three pigment colors
that determine, depending on their proportion, the outward appearance of the iris: brown,
yellow, and blue. Green irises, for example, have blue and some yellow. Brown irises
contain mostly brown. Eye color in animals other than Homo sapiens are differently
regulated. For example, instead of blue as in humans, autosomal recessive eye color in
the skink species: Corucia zebrata is black, and the autosomal dominant color is yellow-
green.[18]


[edit] Changes in eye color
In all populations, children are most commonly born with unpigmented eyes. However,
most babies who have European ancestry have light-colored eyes before the age of one.
As the child develops, melanocytes (cells found within the iris of human eyes, as well as
skin and hair follicles) slowly begin to produce melanin. Because melanocyte cells
continually produce pigment, in theory eye color can be changed. Most eye changes
happen when the infant is around one year old, although it can happen up to three years
of age.[19] Observing the iris of an infant from the side using only transmitted light with
no reflection from the back of the iris, it is possible to detect the presence or absence of
low levels of melanin. An iris that appears blue under this method of observation is more
likely to remain blue as the infant ages. An iris that appears golden contains some
melanin even at this early age and is likely to turn green or brown as the infant ages.

Changes (lightening or darkening) of eye colors during puberty, early childhood,
pregnancy, and sometimes after serious trauma (like heterochromia) do represent cause
for plausible argument to state that some eyes can or do change, based on chemical
reactions and hormonal changes within the body.

Studies on Caucasian twins, both fraternal and identical, have shown that eye color over
time can be subject to change, and major demelanization of the iris may also be
genetically determined. Most eye-color changes have been observed or reported in the
Caucasian population with hazel eyes.[20]

[edit] Eye color chart (Martin–Schultz scale)
Carleton Coon created this chart by the Martin–Schultz scale often used in physical
anthropology.

  I.   Light eyes

       Eyes light and light mixed are 16–12 in Martin scale.

       Light
       Gray, blue, green.
       Light-mixed

         a.  Very light-mixed (blue with gray or green or green with gray)
         b.  Light-mixed (light or very light-mixed with small admixture of brown
             pigment)
 II.   Mixed eyes

       Mixed
       12–6 in Martin scale. Mixture of light eyes (blue, gray or green) with brown
       pigment when light and brown pigment are the same level.

III.   Dark eyes

       Dark-mixed
       6–4 in Martin scale. Brown with small admixture of light pigment.
       Dark
       4–1 in Martin scale. Brown (light brown and dark brown) and very dark brown
       (almost black).
[edit] Amber




Amber eyes / Golden green-brown




Amber eyes in sunlight - displaying an orange color rather than brown

Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery
tint. This might be due to the deposition of the yellow pigment called lipochrome in the
iris (which is also found in green eyes).[21][22] Amber eyes should not be confused with
hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to
comprise many other colors, including green, brown and orange. Also, hazel eyes may
appear to shift in color and consist of flecks and ripples, while amber eyes are of a solid
gold hue. Even though amber is considered to be like gold, some people have russet or
copper colored amber eyes which many people mistake for hazel, though hazel tends to
be duller and contains green with red/gold flecks, like mentioned above. Amber eyes may
also contain amounts of very light gold-ish gray, found in animals like wolves.

The eyes of some pigeons contain yellow fluorescing pigments known as pteridines.[23]
The bright yellow eyes of the Great Horned Owl are thought to be due to the presence of
the pteridine pigment xanthopterin within certain chromatophores (called xanthophores)
located in the iris stroma.[24] In humans, yellowish specks or patches are thought to be
due to the pigment lipofuscin, also known as lipochrome.[25] Many animals such as
canines, domestic cats, owls, eagles, pigeons and fish have amber eyes as a common
color, whereas in humans this color occurs less frequently, more in places like Brazil and
Asia, being rare in other regions.


[edit] Blue




A blue iris
Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths of
light tend to be absorbed by the underlying iris pigment epithelium, and shorter
wavelengths are reflected and undergo Rayleigh scattering.[7] The type of melanin present
is eumelanin.[26] The outer surface of the iris of a blue-eyed person is clear, lacking the
outer layer of pigmentation that is found in brown eyes. Their color is caused by the inner
layer of pigmentation and the semi-opaque fibrous tissues that lie between the two
layers.[27]

[edit] Distribution
Blue eyes are most common in the Baltic Sea region, Northern, Eastern and Central
Europe, and to a lesser degree in Southern Europe and southern Central Asia;
Afghanistan and Pakistan are a notable example.[28] Blue eyes are found in the Levant
and the Middle East, especially amongst the Jewish population of Israel. However, many
modern Israeli Jews are of European Ashkenazi origin, among whom this trait is common
(53.7% of Ukrainian Jews have blue eyes [29]). While blue eyes are thought to be
exclusive to Caucasoid ethnic groups, the manifestation of blue eyes has been
documented in pure-blooded, darkly complected tribal Africans, as well as people of
mixed African and European ancestry; the former, usually the result of genetic mutation
and the latter most often the manifestation of recessive European genes.

A 2009 study suggested that blue eyes were present in Siberia during the Bronze Age; 15
of 25 Andronovo culture specimens (60%) from the Krasnoyarsk area had blue (or green)
eyes.[30]

Y-Chromosome DNA testing performed on ancient Scythian skeletons dating to the
Bronze and Iron Ages in the Siberian Krasnoyarsk region found that all but one of 11
subjects carried Y-DNA R1a1, with blue or green eye color and light hair common,
suggesting mostly European origin of that particular population.[31] In 2005, a 2,500-year-
old mummy of a Scythian warrior found in the the Altai, Mongolia, showed a 30- to 40-
year-old man with blue eyes and blond hair.[32]

A 2002 study found that the prevalence of blue eye color among Caucasians in the United
States to be 33.8 percent for those born from 1936 through 1951 compared with 57.4
percent for those born from 1899 through 1905.[10] Blue eyes are continuing to become
less common among American children, with only one out of every six or 16.6% of the
total population, and 22.3% of the White population having blue eyes.[33][34][35]

				
DOCUMENT INFO